FIG. 7 is a sectional view of one example of a conventional liquid surface level detecting apparatus that uses a reflection of light by a prism. A liquid surface level detecting apparatus 100 shown in FIG. 7 is provided with: an optically transmitting member 102 which has an end (hereafter, referred to as a prism section 101) cut to a predetermined shape and is made of bar-shaped fluoro-resin or glass and the like; a light emitting means 103 for emitting the light to a longitudinal direction of the optically transmitting member 102 toward the prism section 101; a light receiving means 104 for receiving a reflection light which is returned to the longitudinal direction of the optically transmitting member 102 from the prism section 101 after the light emitted by the light emitting means 103 is reflected by the prism section 101; and an IC 105 for measuring a light amount of the reflection light received by the light receiving means 104 and outputting its measured result.
Also, in such a way that the light emitted by the light emitting means 103 can be efficiently received by the light receiving means 104, the predetermined shape of the prism section 101 is defined. For example, it is defined such that the emitted light is totally reflected by the prism section 101, and such that the total reflection light is returned to the light receiving means 104. That is, when a refractive index of the optically transmitting member 102 is assumed to be nt and a refractive index of air is assumed to be ns (ns≈1), in such a way that an irradiation angle of the light from the light emitting means 103 becomes sin θo=ns/nt, a predetermined shape of a low end of the prism section 101 and the installation positions of the light emitting means 103 and the light receiving means 104 are defined.
If a liquid surface level 107 of a liquid 106 is brought into contact with a portion to which the light of the prism section 101 is irradiated, a refractive index of an outside of the prism section 101 is changed (usually, since a refractive index of the liquid 106 is greater than the air, the refractive index of the outside becomes greater), a total reflection angle θo is changed. Consequently, the light from the light emitting means 103 is radiated to an inside of the liquid 106, and the light amount returned to the light receiving means 104 is largely reduced. By measuring the change in this light amount, the fact that the liquid surface level 107 is brought into contact with the low end is detected.
However, in the liquid surface level detecting apparatus 100 of FIG. 7, a liquid surface level detecting unit exists in the prism section 101 at the low end. Thus, once the liquid surface level of the liquid 106 is detected, liquid drops continue to be deposited. Hence, there may be a case that an erroneous operation is induced in the liquid surface level detection or that the liquid surface level detection can not be done. A liquid surface level detecting apparatus that tries to improve this problem is a liquid surface level detecting apparatus shown in FIG. 8 as described below.
FIG. 8 is a sectional view of one example of the conventional liquid surface level detecting apparatus that uses the total reflection light. A liquid surface level detecting apparatus 200 shown in FIG. 8 is provided with: an optically transmitting member 201 which is hollow and made of bar-shaped fluoro-resin or glass and the like; a light emitting means 202 and a light receiving means 203 which are placed in the hollow section therein and composed of optical fibers and the like; a light shielding wall 204 for avoiding a light from the light emitting means 202 from being directly irradiated to the light receiving means 203; and an IC 205 for measuring a light amount received by the light receiving means 203 and outputting its measured result.
Also, if a liquid 206 does not exist in an outside (namely, if the outside is air), in such a way that the light is totally reflected by an outer wall on a side of the optically transmitting member 201, angles of those light emitting means 202 and light receiving means 203 are established. That is, when a refractive index of the optically transmitting member 201 is assumed to be nt and a refractive index of the air is assumed to be ns (ns≈1), in such a way that the light is inputted at an input angle larger than a critical angle θo in which an irradiation angle of the light from the light emitting means 202 is determined by sin θo=ns/nt, the light emitting means 202 is placed. Also, the light receiving means 203 is placed at a position having the same reflection angle as the input angle from the light emitting means 202, in such a way that the total reflection light from the outer wall of the optically transmitting member 201 can be efficiently received.
As mentioned above, if the outside is the air, the light from the light emitting means 202 is totally reflected by the outer wall of the optically transmitting member 201 and received by the light receiving means 203. However, if a liquid surface level 207 is brought into contact with a location (hereafter, referred to as a total reflection unit) where the light of the side of the optically transmitting member 201 is totally reflected, the refractive index of the outside of the above-mentioned optically transmitting member 201 is changed, and the critical angle θo is changed. Consequently, the light from the light emitting means 202 is radiated to an inside of the liquid 206, thereby greatly reducing a light amount returned to the light receiving means 203. By measuring the change in this light amount, the fact that the liquid surface level 207 is brought into contact with the total reflection unit is detected. In the liquid surface level detection using such a total reflection, if the liquid 206 exists in the total reflection unit, the total reflection of the light is not induced, and its reflection light amount is dramatically changed. Hence, the change amount in the light amount of the light receiving means is great, and it is possible to carry out the liquid surface level detection whose precision is high.
Also, Japanese Laid Open Patent Application (JP-A 2000-329607) and Japanese Laid Open Patent Application (JP-A 2000-321116) disclose a liquid surface level sensor for scattering a propagation light, and consequently measuring an attenuation amount of the propagation light, and then detecting a liquid surface level. For example, Japanese Laid Open Patent Application (JP-A 2000-329607) discloses a liquid surface level sensor 300 that uses a scattering light in which the propagation light is scattered as shown in FIG. 9.
This liquid surface level sensor 300 passes and scatters the propagation light through a light scattering member (particle body) 301, and consequently generates the scattering light, radiates the scattering light from a sensing section 302 to outside, and measures the attenuation amount of the propagation light attenuated by the influence of the liquid existing in the outside, and then detects the liquid surface level. Also, besides FIG. 9, for example, as shown in FIG. 10, a configuration in which optically transmitting materials whose refractive indexes are different are placed on the entire U-shaped section is disclosed.
However, the liquid surface level detecting apparatus shown in FIG. 8 carries out the liquid surface level detection through the total reflection. For example, if the liquid surface level detection of a liquid having a high viscosity coefficient is once carried out and that liquid surface level detecting apparatus is again used to carry out the liquid surface level detection, the liquid drops are deposited on the total reflection unit. Thus, the total reflection on the total reflection unit is never induced. Hence, there is a problem that the liquid surface level detection is impossible unless the liquid drops are removed. Also, if oil film and contaminant are deposited on the total reflection unit, similarly, there is a problem of a possibility that an erroneous operation is induced.
Also, in the liquid surface level detecting apparatus shown in FIG. 8, since the establishment of the critical angle to define the total reflection is carried out in the prism section and the total reflection unit, it is necessary to establish the various conditions such as the angle of the prism section, the installation positions and installation angles of the light emitting means and the light receiving means, the distance from the light emitting means to the light receiving means (namely, the distance in the longitudinal direction of the light shielding wall), and the light emitting angle and the light receiving angle, and the like. For example, such as the case that even one omission from those conditions disables the liquid surface level detection and the like, there is a problem that the durability against the deterioration in the liquid surface level detecting apparatus caused by the various usage conditions, such as flaw and damage resulting from shock from the outside, acid-base property and the like, is very weak. Also, since those various conditions need to be established, there is a problem that the manufacturing of the liquid surface level detecting apparatus becomes precise and difficult.
Also, the liquid surface level detecting apparatus shown in FIG. 8 is established, for example, in such a way that with regard to the initial state at which the outside of the liquid surface level detecting unit is the air, the total reflection is induced, and within the predetermined detection target (liquid), the total reflection is not induced. On the other hand, such a detection target is different depending on a usage environment. Thus, there is a problem that depending on the establishment of the critical angle θo, although the liquid having a high refractive index can be detected, the liquid having a low refractive index cannot be detected. Also, the conventional liquid surface level detecting apparatus has a problem that an erroneous operation is induced if the detection target is colored liquid.
Also, the liquid surface level detecting apparatus shown in FIG. 8 has the following problem. For example, it can detect a boundary between two layers of air and liquid. However, if it tries to detect a boundary between three layers of air, oil and water, it must use two kinds of liquid surface level detecting apparatuses. That is, one is a liquid surface level detecting apparatus which is established such that the change of the presence or absence of the total reflection is induced on the boundary between the air and the oil, and the other is a liquid surface level detecting apparatus which is established such that the change of the presence or absence of the total reflection is induced on the boundary between the oil and the water. By the way, in this specification, a boundary between a liquid phase and a gaseous phase and a boundary between different two liquid phases are referred to as a liquid surface.
Also, the liquid surface level sensor shown in FIG. 9 has the structure that the light emitted from the light emitting means is directly inputted to the light receiving means, and the light receiving means receives the considerable quantity of propagation light, even before the liquid surface level detection. The light directly inputted to the light receiving means from this light emitting means does not contribute to the liquid surface level detection and further greatly reduces the precision in the liquid surface level detection. In particular, at the time of the liquid surface level detection, if the scattering light amount radiated to the outside from the sensing section (namely, the attenuation amount of the propagation light by the light receiving means) is small, a variation rate of the light reception amount of the light receiving means (=the light reception amount after the liquid surface level detection/the light reception amount before the liquid surface level detection) becomes very small. Thus, it is very difficult to measure that variation. Hence, there is a problem that this is not practical.
Thus, in particular, if a distance L of the sensing section is made shorter in order to improve the precision in the liquid surface level detection, the scattering light amount radiated to the outside from the sensing section is inevitably reduced, thereby further reducing the variation rate of the light reception amount of the light receiving means. Hence, this liquid surface level sensor has a problem that unless the distance L of the sensing section is set to be considerably large, the liquid surface level detection is impossible and the precision in the liquid surface level detection is very low.
Also, as shown in FIG. 10, the liquid surface level sensor, in which the optically transmitting materials having the different refractive indexes are placed on the entire U-shaped section and the light is radiated from the U-shaped section, has the structure that the propagation path of the light is especially curved, and most of the propagation light is leaked as the scattering light to the outside, which results in the extreme reduction in the light reception amount of the light receiving means. Also, moreover, even if the liquid surface level is detected, it is impossible to accurately specify the liquid surface level. Thus, only the liquid surface level detection of the very low precision can be done. Hence, there is a problem that this is not practical.